What is the best way to prepare and paint new exterior fiberglass doors?

Wash it down with a pre-paint prep solution or mineral spirits. Paint all trim and non-flat areas with a brush and then the flat areas with a roller. You could spray it if you have that equipment. You can add Flotrol to your paint to get a smoother finish. Read this for more info on Flotrol and spraying. Wagner painters - Tool Shed Forum - GardenWeb

Nice answer. I tell basically the same to most of the people I sell doors to al Lowe's. Some people will just apply paint without cleaning. Then they return the door as faulty because the paint did not stick. Heaven forbid it was their fault.